I have done the same. Tried not restored from backup or iCloud and have had no discernible improvement. I’m at a loss.
At this point I am starting to wonder if there is something in my iCloud profile that’s corrupted that causes background issues. Since the problem seems to keep following me and the only constant is logging into my iCloud account. But I have no way of digging into that or testing my hypothesis without abandoning my whole iCloud account.
You may really be on to something. Or there is a bug is iOS 18 with iCloud backups. I only think this because I had been having an issue where some of my photos would not let them be sent in iMessage. I could email them, just could not send in iMessage. I think iOS 18 messed some users iCloud files up.